1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring the volume of a body or other solid object. Still more particularly, the present invention relates to a body volume measurement technique using a soft-sided bag.
2. Background of the Invention
A complete evaluation of a person's physical condition includes a physical examination as well as other assessments. For many patients it is desirable to assess the patient's percent body fat, bone mineral content, and volume of water. For example, by measuring or estimating the mineral content of a person's bones, various skeletal disorders such as osteoporosis can be monitored and appropriate treatment can be prescribed. Further, for long duration space flight in which an astronaut's muscles and bones naturally atrophy from reduced usage, it is important to evaluate, and preferably minimize, the magnitude and extent of the atrophy. A determination of body composition aids in this evaluation.
A crude body composition model of a person includes a fat component (also referred to as a "compartment") and a lean body mass component (non-fat mass). A three-compartment model has also been suggested for modeling body composition. The three-compartment model includes a fat compartment, a dry lean mass compartment, and a water compartment (typically referred to as "total body water").
Various techniques have been suggested for measuring body composition. For example, in one technique, measurements of the body's electrical impedance are used to estimate total body water using regression equations.
The two-compartment model comprising lean body mass and fat mass usually is determined by measuring a patient's total mass (typically measured in units of kilograms) and total body volume (measured in units of liters). On the ground, mass is determined from a recording of a person's weight. Determining mass in this manner in the near weightless environment in which an astronaut works does not work. Numerous other techniques for measuring an astronaut's mass have been suggested and attempted.
The standard technique for measuring a patient's total body volume has been underwater weighing (also referred to as "hydrostatic weighing"), a technique based on Archimides' Principle. Using this technique, the difference between a subject's weight out of water and the subject's weight submersed in water is measured. The subject's body volume equals the volume of water displaced by the subject once submersed in the water. The volume of water displaced is determined by calculating the volume of water corresponding to the difference in the person's weight in and out of water. Although generally accurate, the underwater weighing technique requires complete submersion of the person in water and maximal expulsion of air from the person's lungs and measuring the residual volume left in the lungs.
Submersion of some patients may be impossible because of the patient's condition. Subject movements, incomplete lung emptying, and variability in lung measurements affect the accuracy of the resulting measurement. Further, some patients are afraid of submersion in water. Additionally, this technique cannot be used during space flight.
Hard-sided volumometers have been used to measure total body volume. These devices are based on Boyle's gas law in which the product of the volume of a gas multiplied by its pressure is a constant value at a fixed temperature. A hard-sided volumometer is a rigid chamber large enough to hold a patient. The person's body volume is measured by measuring the pressure in the chamber while the volume of the chamber is varied, and applying Boyle's Law to the resulting pressure measurements. Hard-sided volumometers have not been used extensively because their accuracy is usually less than desirable. Further, hard-sided volumometers generally are not portable making their use for many patient's impractical. Where weight and volume are at a premium such as in space, hard-sided volumometers are too bulky and heavy to be feasible. Finally, underwater weighing systems and hard-sided volumometers are very costly.
Thus, an accurate, portable, and inexpensive volumometer for measuring a person's total body volume is needed. Such a device could easily be used in a variety of situations including hospitals, doctors' offices, health clubs, space flight, and the like.